Here are some common problems and potential causes that are related to engine fuel components. Included in this text, as an aid to understanding the hardware, are functional descriptions of the RSA Bendix Fuel Injector, Flow Divider and Nozzles, the Marvel-Schebler Carburetor and Teledyne Continental Fuel Components.

RSA Fuel Injector Functional Description

All RSA type fuel injection systems are based on the principle of measuring engine air consumption by use of a venturi and the resulting airflow forces to control fuel flow to the engine. Fuel distribution to the individual cylinders is accomplished by use of a flow divider (i.e "spider") and air bleed nozzles.

The principal of operation is as follows. Air flow consumption of the engine is sensed in the injector by means of the impact tubes located in the inner venturi area. A known venturi size will meter a predetermined amount of air and generate a velocity pressure at the tubes that is transmitted to both sides of the air or outermost diaphragm located on the regulator section of the injector. The regulator is defined as the large diameter cover on one side of the injector and consists of the air and fuel diaphragms, connected to a ball servovalve, and controlled by a bellows and springs.

Thus, opening of the throttle valve on the injector causes an increase in engine air consumption and a resulting pressure increase in the venturi. This higher pressure is sensed by the air diaphragm via the impact tubes that results in movement of the ball servovalve in the opening or increased flow direction. The flow to the engine then increases until the pressures on both sides of the fuel diaphragm balances the regulator system to bring it into equilibrium.

This is a simple closed loop control system in which a imputed change (throttle adjustment) results in an output difference (fuel flow) that is fed back to the system (at the fuel diaphragm) to control the operation (engine speed). This scheme results in the optimum fuel-to-air ratio being maintained for correct engine performance. On fuel injectors with an external bellows, fuel or mixture is automatically controlled by the affects of altitude and the resulting movement of the evacuated bellows that is hydraulically linked to the regulator components.

The fuel metering section on the injector consists of the inlet fuel strainer, manual mixture control valve, and an idle valve. The idle valve is connected to the throttle valve by means of a link which incorporates a "star wheel" for idle adjustments. The lever movements made with this adjustment are minuscule and not easily visible due to the slightly different thread pitch on both sides of the wheel. The idle speed is adjusted by the threaded screw on the stop at the throttle lever. The mixture control valve is a simple on-off assembly that is manually adjusted by its lever.

The function of a certified repair station is to calibrate the fuel injector to engine standards by adjusting and fixing the regulator with springs and threaded components while flowing the unit on an approved test bench.

Flow Divider Description

The metered fuel is delivered by the injector to a pressurized flow divider which distributes the fuel to the individual cylinders via dedicated lines and nozzles. The flow divider is a simple spring loaded valve that is sealed to atmosphere by a diaphragm. The valve movement and resulting fuel flow is strictly regulated by pressure acting on the valve. At idle, the fuel pressure must build to overcome the spring and diaphragm load in order to move and open the fuel ports. Metering at this speed is primarily a function of the flow divider but as flow increases and the valve opens, the fuel nozzles become the primary system metering components.

Air Bleed Nozzles

The fuel discharge nozzles at the cylinder each contain a calibrated jet that is sized according to the system flow requirements of the engine. All nozzles used in Bendix fuel systems are designed to flow the same (32 PPH @ 12 PSI) and all have an "A" stamped on one hex that designates the location of the air bleed hole. When installing, torque so that the "A" is located down and then the hole is pointed up so that residual fuel is prevented from escaping.

Bendix Fuel Injector Troubleshooting

PROBLEM

PROBABLE CAUSE

REMEDY

HARD STARTING

Technique.

Refer to aircraft manufactuer's recommended starting procedures.

Flooded.

Clear engine by cranking with throttle open and mixture control in ICO.

Throttle valve opened too far.

Increase amount of priming.

ROUGH IDLE

Mixture too rich or too lean.

Confirm with mixture control. A too rich mixture will be corrected and roughness decreased during lean-out while a too lean mixture will be aggravated and roughness increased. Adjust idle to give a 25-50 rpm rise @ 700 rmp.

Plugged nozzle if high fuel flow is accompanied by loss of power and roughness.

Remove and clean nozzles in Acetone, MEK, hydrocarbon cleaning solvent, or a chlorinated solvent equivalent to chlorothene is recommended.

Faulty gage.

Criss-cross gages and replace if necessary.

Injector out of adjustment.

Replace injector.

STAGGERED MIXTURE CONTROL LEVERS

If take-off is satisfactory, do not be too concerned about staggered mixture control levers because some mis-alignment is normal with twin engine installation.

Check rigging.

POOR CUT-OFF

Improper rigging of aircraft linkage to mixture control.

Adjust.

Mixture control valve scored or not seating properly.

Eliminate cause of scoring (usually burr or dirt) and lap mixture control valve and plug on surface plate.

ROUGH ENGINE (TURBO CHARGED) AND POOR CUTOFF

Air bleed hole(s) clogged.

Clean or replace nozzles.

Marvel-Schrebler / Facet Float Carburetor Troubleshooting

PROBLEM

PROBABLE CAUSE

REMEDY

ROUGH IDLE

Mis-adjusted idle mixture.

Re-adjust idle mixture per engine manual.

Bad or leaky primer.

Disconnect and cap off to test for problem.

Cracked primer lines.

Inspect all joints and connections. Repair as necessary.

Intake manifold leaks.

Pressurize and test.

POOR IDLE CUT-OFF

Mixture linkage not full travel.

Re-adjust idle mixture per engine manual.

Mixture valve being pulled up by misaligned mixture cable.

Re-align mixture cable straight with mixture lever.

Leaky primer

Disconnect and cap off to test for problem.

Idle speed adjusted too high.

Re-adjust idle speed per engine manual.

CAN'T ADJUST IDLE

Leaky primer

Disconnect and cap off to test for problem.

Cracked primer lines.

Inspect all joints and connections. Repair as necessary.

Intake manifold leaks.

Pressurize and test.

RUNS RICH (LEANING MIXTURE HELPS PROBLEM)

Leaky primer

Disconnect and cap off to test for problem.

Mis-adjusted idle mixture.

Adjust to obtain 25-50 RPM rise at ICO.

RUNS LEAN (CARB HEAT HELPS PROBLEM)

Cracked primer lines.

Inspect all joints and connections. Repair as necessary.

Intake manifold leaks.

Pressurize and test.

Mis-adjusted idle mixture.

Adjust to obtain 25-50 RPM rise at ICO.

STUMBLES ON ACCELERATION

Leaky primer

Disconnect and cap off to test for problem.

Idle mixture adjusted too rich.

Adjust to obtain 25-50 RPM rise at ICO.

Pump linkage mis-adjusted.

Re-adjust pump linkage.

The Marvel-Schebler Carburetor Idle Circuit

There are four small holes in the throat of the Marvel-Schebler carburetor that provide the idle function for the engine. The top hole is the fuel discharge orifice and the bottom ones are air bleed holes.

These holes carry out different functions depending on the position of the throttle plate. When the plate is fully closed and the pressure under it at the bottom holes is higher than the top one on the other side, the bottom holes act strictly as "air boost" circuits. That is, they utilize this higher pressure to atomize and force fuel out of the top hole to maintain an engine idle setting.

As the throttle is opened and the air consumption of the engine increases, more fuel is needed but the differential pressure between the bowl reservoir and the top discharge hole decreases. However, at the same time this seemingly detrimental sequence unfolds the plate has moved past the second hole, placing it in a low pressure region and transforming this orifice into a needed fuel jet. This continues for the third hole, until the plate is opened sufficiently to allow the interaction of the air through the venturi to discharge sufficient fuel from the main nozzle.

The idle screw on the side of the carburetor throttle body has a needle point that extends into the top orifice. Adjustment opens and closes the circuit to calibrate the fuel discharge for proper engine idling.

The timing of the operation of the idle circuit is critical to the performance of the engine as the throttle is advanced. Without the holes there would be a pronounced lean condition that would manifest itself as a severe engine stumble if speed is increased too rapidly.

The Marvel-Schebler Carburetor Theory of Operation

There is some misconception about how fuel is discharged from a standard float carburetor. A belief that the action of the downstream pistons "sucks" the fuel out of the carburetor is incorrect. The proper understanding begins by noting that the outlet of the main discharge nozzle is located in the center of the venturi and that fuel in a carburetor only flows when subjected to a differential pressure.

When the throttle is advanced, opening the plate in the carburetor, the amount of air through the venturi is increased. The venturi geometry in turn significantly increases the velocity of the air and creates a region of low pressure in the venturi center or at the outlet of the nozzle. Since the fuel in the carburetor bowl remains at atmospheric pressure, which is now higher than the pressure in the venturi, fuel is actually forced out of the bowl to the nozzle. The movement of the air in the venturi then assists in atomizing the fuel for combustion.

Thus, the operation of a carburetor on an engine constitutes a simple closed loop control system in which a imputed change (throttle or accelerator pump adjustment) results in an output difference (fuel flow) to control the operation (engine speed). In a float carburetor this causes the bowl to drain and the float to open the fuel inlet. The feedback mechanism is the actual engine speed that will only consume the amount of fuel needed to sustain its operation and maintain the float in a steady state position. This scheme results in the optimum fuel-to-air ratio being maintained for correct engine performance.

Teledyne Continental Fuel Components

This is a summary of TCM Service Bulletin SID97-3A for fuel system adjustment. Refer to it for detailed instructions.

The typical TCM fuel system consists of a fuel pump, metering unit as a part of the throttle body, flow divider, lines and nozzles. The operation is based on pump pressure adjustments at idle and full throttle and flow adjustments on the air throttle metering unit. Fixed orifice pumps are distinguished by a single adjustment of the relief valve that is located on the back centerline of the unit. Variable orifice pumps have an additional adjustment located on an aneroid or bellows or on the side of the pump if an aneroid is not present.

It is important to note that adjustments to this system can only be accomplished successfully with an engine that is free of non fuel system problems such as faulty ignition timing and compression leaks.

Idle pump pressure should be adjusted prior to idle mixture setting. On naturally aspirated engines with pumps that have a variable orifice follow this procedure:

1. Tap into the unmetered pressure line that runs from the pump outlet to the metering unit with a calibrated, ambient pressure gage (vented to atmosphere).

2. Check that the idle adjustment screw on the metering unit is backed out flush with the lever surface so as not to contact the stop. Position the throttle to fix the engine idle speed RPM.

3. Check that the mixture control lever is in the full rich position and adjust the pressure relief screw on the centerline of the pump to obtain the required idle pressure limits. Adjust in or clockwise to increase pressure and out or counter-clockwise to decrease.

4. Adjust the idle mixture screw on the metering unit as required after checking for the recognized 25-50 RPM rise when advancing the mixture control to the closed position.

5. Check full throttle performance by monitoring metered pressure or flow with a gage tied into the metering valve outlet line or utilizing the cockpit gage. Advance full throttle and adjust the variable orifice screw on the side of the pump to obtain the desired setting. Adjust in or clockwise to increase pressure and out or counter-clockwise to decrease.

Step 5 can only be accomplished with pumps that have a variable orifice, some early TCM pumps do not have this feature, requiring all adjustments to be made with the single relief valve setting. On turbocharged engines, this adjustment is on the top of the aneroid housing and is modulated after loosening of the jam nut. Because this threaded stem is a part of the internal bellows, adjust clockwise to decrease pressure and counter-clockwise to increase it.

There is no adjustment to the TCM Flow Divider and its operation is similar to the Bendix unit. The major difference is the existence of a seal in the valve seat that does aid in providing fuel shutoff on mixture control closing.

It is important to note that whereas virtually all Bendix nozzles flow the same, this is not the case with TCM. There is a wide variety of flow tolerances on similar looking TCM nozzles, they are distinguished by a letter marked on one hex flat. Refer to the engine application listing for clarification.

Problem With Rough Idle (Carbureted)

Primary cause of a rough idling engine due to a faulty fuel system is a mis-adjusted mixture. A lean running engine will display stumbling on throttle advance while a rich engine will emit unburned fuel or black smoke. Adjust mixture accordingly and also reset the idle speed. If the problem persists with a carbureted engine, it could be an indication of a sinking float either by a defect in a metal constructed one or the use of a composite float that is prone to fuel absorption. Composite floats should be replaced.

Another feature to check is the stability of the venturi, a loose fitting venturi is a serious problem, not only at idle, but throughout the operating range of the engine. This is the reason for the directed removal of all two-piece venturis and its replacement with a single-piece one.

Non fuel component sources of a rough idle are ignition system problems including fouled spark plugs and worn leads and induction air leaks.

Lean Condition at Full Throttle (TCM)

Teledyne Continental Fuel Metering Units do contain a fuel circuit which allows fuel to drain back to the fuel pump. This is indicated by correct unmetered (pump to metering unit) fuel pressure and low metered pressure (out of the metering unit). The cause is fuel leaking by the mixture control valve and can be fixed by lapping this valve.

Problem With Rough Idle (TCM)

A specific area to evaluate with TCM fuel components when experiencing a rough idle is the integrity of the fuel pump. There could be contamination in the check valve of the pump or loose through bolts that keep the pump assembly together. This should be evaluated by a certified facility.

Fuel Dripping From Throat of Carburetor

This problem is usually associated with the float and, more specifically, the needle and seat. Often times a small piece of contaminant can find its way to the seat area and prevent a positive seal by the needle. For Stromberg/NAS model carburetors utilized on "tail draggers", care must be taken to control the fuel level in the bowl to accommodate the slanted mounting of this unit on the engine to prevent overflow leakage. There is a modification to this carburetor per Service Bulletin #73 dated 6/58 that removes a potential leaking air bleed hole located low in the throat by plugging and re-drilling a new one further up. This should only be done by a certified facility.

Another cause of throat leakage could be a sinking float as described previously that will also contribute to an overall rich engine condition.

Poor Or No Idle Cutoff

This situation is almost exclusively a fuel component problem. The proper work around for a Bendix Fuel Injector is described previously that details a scored mixture control plate that requires lapping. For TCM hardware, the shutoff capability of the flow divider is key and repair of this unit should only be accomplished by a certified facility.

On Marvel-Schebler carburetors, idle cut off is completed by the mixture control valve that extends through the throttle body and nests in a "seat" that is part of the bottom of the bowl. The male portion of the mixture control valve is a half moon shape that opens and closes the fuel circuit as it is rotated. Often times the male valve wears beyond a point where the close diametrical tolerance required to maintain a positive shutoff is lost and leakage occurs. The only solution is to replace the mixture control valve. Usually this is the item that wears and not the seat, however, if the seat is damaged the entire bowl must be replaced.

A simple check of the fuel shutoff capability for a carburetor is to fill the bowl with fuel and, with the mixture open, tilt the carburetor so that fuel dribbles out of the nozzle. A good mixture control valve will immediately shut off the fuel when rotated and not display any residual dripping.

Off Idle Stumble

This problem is described as the hesitation that takes place as the engine attempts to transition from an idle speed to a higher operating condition.

On carburetors, possible causes are a loose venturi, malfunctioning accelerator pump because its internal leather seal is worn or the linkage that attaches to it is worn, air leakage at the bowl gasket and/or throttle shaft or a loose or worn mixture control lever.

On RSA Fuel Injectors, check the idle mixture and/or idle speed adjustments for the correct engine setting. If the engine will only run properly with the electric boost pump operating it could mean that air is being ingested into the engine driven pump and disrupting the operation of the injector. Check for this problem by installing a clear line between the injector and flow divider.

High Temperature on a Single Cylinder

Most likely on fuel injected engines in which the fuel circuit to this cylinder is restricted either at the nozzle, the line or flow divider. Confirm by doing a distribution check by flowing each nozzle simultaneously into same volume containers. Keep the fuel system of injector, flow divider and nozzles intact, open the mixture control valve full and operate the electric boost pump for a period of time to sufficiently fill the containers. Compare the volume level of each.

If there is a deficient line attempt to isolate by swapping nozzles, lines etc. and repeating this test. Do not overlook the fact that Teledyne Continental fuel lines have a smaller I.D. at the ball sealing surfaces on their ends than Lycoming and a distribution problem could be introduced by incorrectly mixing these lines.

Disclaimer

Information contained in these pages are for reference only. D & G Supply is not responsible for any problems resulting in its use and does not warrant the information for its application.